Misregulated signaling pathways can lead to defects in cell growth and development and thus to cancers and birth defects. The mechanism of this misregulation can best be understood in the context of normal signaling; therefore the goal of the work describe here is to understand the normal function in a signaling pathway triggered by members of the Wnt ligand family. Activation of the Wnt pathway is important during development, aberrant activation has been implicated in the genesis of both colon cancer and melanoma. The Wnt signal cascade starts at the cell membrane when a secreted Wnt proteins binds to its cell surface receptor and initiates a series of reactions that propagate the signal through the cytoplasm to the nucleus. In the nucleus, the Wnt signal directs changes in gene expression via a complex consisting of beta-catenin and one of four Lymphoid Enhance Factor or T Cell Factor proteins (collectively, LEF/TCFs). Unfortunately, very little is known about how the beta- catenin. LEF/TCF complex forms or how it regulates gene expression. Based on our data and work from other groups, we have formulated a model for the activities of this complex. These sequences differ among the four LEF/TCFs and thus a test of our model will yield insights as to how Wnt can direct complex, yet finely tuned effectors through different beta-catenin. LEF/TCF complexes. Specific Aim 1 describes experiments to define a transcription activation domain of beta-catenin and identify proteins that bind to this region. If the state hypothesis is correct, recruited proteins should participate in transcription activation. Aim 2 describes experiments to define a region in LEF-1 that confers enhancing activities on the complex. This same domain may also recruit the transcription repressor Groucho, a test for its ability to antagonize the actions of beta-catenin will be examined. Aim 3 describes experiments to assess the role of LEF/TCF DNA binding domain in the recruitment of beta catenin and, paradoxically, an antagonizing nuclear co-activator to promoters.